


TN 295 

.U4 




I 






H 















■ 



^H 






m*j< 



■ ■ ■ I 

■ 

( Ihiflft'W !,->.' rt»/ :■> ■ > .-ft; ■ • ■ I ■ ■ 









%/ 




• v.^ 






:. V^ 






X <? 



»bv* 









^•y %-^v v^-y %*^v v^v 















: ^"^ -.^W.' ./"V '.., 



*• %S •■ 



X'^'Y v-^'X X'^*Y v-w*«^ V^ 






^\*> °°*^V 







* *> 



v »!,•"* c\. 



*<u** -*^»' ^ 



>?* ..i 



OB* <^ .., 



^•y v^y.. 



• "^ :'JS 






•o 

»4q 



V 



• : ^1 



^ / 



L^^. 



v^ .-ate- %,♦* .-^fefe\/-.-ate> %^ ^ 



vV^, 



Y^ 1 



vV^, 















0^ o«^».^o. 






.°^ 



i 



A* *^J^^* A «. *.£.,£..• «P^ A* >«4RW^" t ^ «. *»^^.« ^ A* 






^^ 






• * ^ 













*° ^ y ^ -Mm* .#"%. • 



«5°a 












a Hq, 






'•' <** 







'V 







A <* 'o.l 

W ... <«> 














o* ^. *?Kv\a <^ *•.»* ^ ho ^ 



»bv* 






V%^*\/ °o*^- / 



y^ 







& .^V>jlV -% >' 










••' -^. 






^ ^- 4Va\ V./ •' 






q,^ * . , 1 • aP 









J i»v 



W 



> _**V : .1B§V <?%> l ^W: _**%, \mg: J>% °»' ***■ 



& ^ VSR** > v ^> •: 






v« 






.V . o » ■ 



I ^ - 






■q. */,,•' a?' «5 



* V 






%>** 



V o 1 



^ .& •vs^r- ^ ** * 












iO v .*«il<v *> 



0* ^ 



*• ^ & *A ' 

w 



q. •„, 



°- /♦*% ' %ifc o - X^\> * s4Mk>* /**m 






^ / V 



** ... 







v v /r 



%. 




A J 









v v ,•* 






r <u* v .' 



«f ^ v*OpV * v ^ °* 
















'Co ,* 







X/&\ x.aatx y,«feX y,si&-x y.-: 






A ^ p 



? ^ O v : si^^y o - ■ 






■"l* . s * • * 'O 




i9 v .•• 



v> " « " V 






lV^ 






V 





w 








:• ° 4 .♦* 



^ 







» iL 



^^ 











v »!,•»*. q, 



? v .* 

























c) ♦/TvT» A 



V *♦ 







* %> d ••^fl£- ° ^^ fgiisk* ** c *"^a£- ° 4 *£(!%?>■> 







S. A ^^J^L^V. .v 



l- >, .^ .*<geii'- ^ ^ /j 









r- 






% ''^VT 4 -^ 



' - 



/ ^% °^M : y^ '-ai^ ^*- 






A 




%>*«* -*iSBS-: ^^ ;^i%^: ^« 



v^ 






4 ^ * *\ % * 

.0* .o^^^b, -V % m ^ 







S V 



y i? 



**o« 



■bv* 



^«fc..%. .y.^°:.X y..ittfe,^. ,/.;i-i:-X ^,^fc-^ 






Vv^ 













q, "•'TVi* A 



<* *••»* ^ G N ^. *?TX* .A 










•V 



> ^ 




4 0, 



? ^ ^ 




.A' .f -, '•*. r>^ ,..' 







J£J 8960 



Bureau of Mines Information Circular/1983 



Microseismic Instrumentation 
Developments 

A Tape-Triggering System and Energy Analyzer 
By Bernard J. Steblay 




UNITED STATES DEPARTMENT OF THE INTERIOR 



(MLM_&&*. fr ^fM^ g) 



Information Circular 8960 



Microseismic Instrumentation 
Developments 

A Tape-Triggering System and Energy Analyzer 
By Bernard J. Steblay 




UNITED STATES DEPARTMENT OF THE INTERIOR 
William P. Clark, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 



T 



f\0 J 



Library of Congress Cataloging in Publication Data: 



Steblay, Bernard J 

Microseismic instrumentation developments. 

(Information circular / United States. Bureau of Mines ; 8960) 

Bibliography: p. 12. 

Supt. of Docs, no.: I 19.4/2:8960. 

1. Mine safety— Equipment and supplies. 2. Seismology— Instru- 
ments. 3. Digital-to-analog converters. 4. Analog-to-digital convert- 
ers. I. Title. II. Series: Information circular (United States. Bureau 
of Mines) ; 8960. 



TN295.U4 622s [622'.8] 83-600282 



oL contents 

Page 

Abs tract 1 

Introduction 2 

Tape-triggering system 2 

Recording section 2 

Reproducing section 4 

\. Energy analyzer 6 

Analog-to-digital conversion 6 

Arithmetic processing. 8 

Application. 9 

Conclusions 11 

Tape-triggering system 11 

Energy analyzer 12 

References 12 

ILLUSTRATIONS 

1 . Tape-triggering system and analog reproducer system 3 

2. NRZ recording signal format 4 

3. Timing channel format 5 

4. Time of day information as presented on output record 5 

5. Energy analyzer system 6 

6 . Waveform digitization 7 

7. Energy analyzer arithmetic processing 10 

8 . Energy system circuit modifications 11 

TABLE 

1 . Internal energy processing 8 



A 






UNIT 


OF MEASURE ABBREVIATIONS USED IN 


THIS REPORT 


dB 


decibel pF 


microfarad 


ft 


foot )JS 


microsecond 


h 


hour ns 


nanosecond 


Hz 


hertz pet 


percent 


ips 


inch per second pF 


picofarad 


kHz 


kilohertz s 


second 


min 


minute V 


volt 



MICROSEISMIC INSTRUMENTATION DEVELOPMENTS 

A Tape-Triggering System and Energy Analyzer 

By Bernard J. Steblay ] 



ABSTRACT 

Two instruments have been constructed that extend microseismic data 
collection and processing capability for Bureau of Mines research in 
rock burst, coal bounce and outburst, and roof fall monitoring. The 
first instrument is a 13-channel tape-triggering system. This system 
allows better utilization of instrumentation tape recorders by collect- 
ing data digitally, turning the tape on only when valid data are pres- 
ent, and increasing the dynamic range of the recorder by using digital 
data. A digital-to-analog converter is used to provide analog output of 
the recorded digital data. The instrument records a much larger time 
slice of transient events than do current commercial devices. The sec- 
ond instrument is a microseismic energy measurement device. This device 
uses recent developments in integrated circuits to overcome the dynamic 
range and accuracy limitations of previous instruments. The analog sig- 
nal is converted to a digital one, and then high-speed multiplication 
techniques are used to square the amplitude value in real time and inte- 
grate it. 



1 Mechanical engineer, Denver Research Center, Bureau of Mines, Denver, CO. 



INTRODUCTION 



The first instrument addressed in this 
paper deals with the frequently en- 
countered problem in data collection of 
recording analog data that consist of 
randomly occurring transient events. If 
an analog tape recorder is run continu- 
ously, it places time constrictions on 
the data collection and also requires 
laborious scanning of a long tape to find 
a few pieces of significant data. This 
problem occurs for the Bureau of Mines in 
collecting microseismic data. The tape- 
triggering system is a solution to these 
problems. It triggers the tape only when 
valid data are occurring, but because it 
first records the data in its solid state 
memory, it does not lose any pretrigger 
data. It can operate unattended for long 
periods since it requires servicing only 
when the tape fills up. It operates with 



ordinary frequency modulation (FM) analog 
recorders but greatly increases their 
dynamic range because of its digital en- 
coding. Using 24,000-byte memory per 
channel allows long transients to be 
recorded. 

The second instrument addressed deals 
with the data collection problem of 
determining the energy of an incoming 
analog signal. This energy estimation is 
a valuable piece of data for rock stabil- 
ity determination. Analog or quasi-analog 
approaches such as voltage-to-frequency 
conversion for acoustic emission measure- 
ment have serious limitations in dynamic 
range and accuracy (l). 2 The energy mea- 
surement system discussed in this report 
overcomes such limitations by using a 
fully digital processing approach. 



TAPE-TRIGGERING SYSTEM 



The tape-triggering device was con- 
ceived and specified by the Bureau of 
Mines Denver Research Center and designed 
and constructed under a Bureau contract 
(_2 ) . Its two major sections are the 
event recorder and control unit and the 
event reproducer unit (fig. 1). The 
units are designed to work with an FM 
instrumentation tape recorder with a min- 
imum bandwidth of 40 kHz (3). 

RECORDING SECTION 

The event recorder and control unit 
first filters the incoming data using a 
low-pass filter set at either 1 or 10 
kHz. The filtered analog signal is then 
converted to a 12-bit digital signal at 
an 83.3-kHz sample rate. The sample rate 
was chosen to provide good analog resolu- 
tion at the output of the event repro- 
ducer even at the upper bandwidth of 10 
kHz. The 12-bit resolution provides a 
total dynamic range for the bipolar input 
of greater than 70 dB , which exceeds the 
typical performance of an analog FM 
recorder by more than 16 dB. 

The digitized amplitude values are then 
stored in a 294,912-bit (24,576-word) per 



channel memory. This large per channel 
memory allows the storage of nearly 0.3 s 
of signal in memory at any given time. 
This overcomes the time limitation caused 
by the short memories typically found in 
commercial transient recorders. The 
event that the author felt made such a 
device commercially practical was the 
development of 64,000-bit charge- 
coupled-device (CCD) shift register mem- 
ories, but these could not be supplied 
in time for construction. Substituting 
64K dynamic access memory proved 
satisfactory. 

The memory of the event recorder is 
divided into pretrigger and posttrigger 
sections. Data are continually being 
recorded by the memory. As each new sam- 
ple is collected, the previous sample is 
shifted one memory cell down the chain. 
The trigger criteria are controlled by a 
microprocessor. If a valid trigger 
occurs, the 2,500 samples that were col- 
lected just prior to the trigger are 
saved. The remaining 22,076 words of the 

2 Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



FIGURE 1. - Tape-triggering system and analog reproducer system. 



memory chain are then filled with post- 
trigger data. Thus about 0.03 s of data 
prior to the trigger is saved for analy- 
sis. This information is often crucial 
for making first-arrival analyses. 

The microprocessor controls both the 
threshold criteria and the valid event 
criteria. The threshold level is se- 
lected by front panel switches and ranges 
from 0.4 V to 2 V in 0.2-V increments. 
If the threshold level on any channel is 
exceeded, the event recorder begins re- 
cording all of the other channels simul- 
taneously. When the memory chain is 
filled, the microprocessor determines 
whether enough channels exceeded their 
thresholds to constitute a valid event. 
This channel requirement number, which is 
settable from 2 to 5, insures that a 



spurious single event on one or more 
channels will not trigger the system. In 
the case of a mining application, a typi- 
cal spurious event would be drilling near 
a geophone. Once it has been determined 
that sufficient channels have valid data, 
the recorder is remotely triggered. When 
the recorder tachometer indicates proper 
speed, or a 5-s default time is reached, 
insuring that proper tape speed has been 
reached, the 0.3 s of digital data for 
all channels are recorded on the tape. 

The digital data are encoded for the FM 
tape using a digital standard nonreturn- 
to-zero (NRZ) signal clocked out at 
40,000 Hz (fig. 2). Since each channel 
was sampled using a 12-bit-wide word 
24,576 times, 294,912 bits must be 
written to the tape. The write time is 



Sample 

NRZ data 

stream 



r 



NOTE : NRZ format requires the dc levels of FM record. 

Bi <f> L format would have to be used with direct record 

FIGURE 2. - NRZ recording signal format. 



then about 7.5 s. Each channel is 
written to its respective tape channel 
simultaneously. If the tape is initially 
stopped, the total time to accelerate to 
speed, write data, and decelerate to a 
stop is about 12 s. If a second tran- 
sient or group of transients occurs be- 
fore the tape is stopped, the transfer of 
data will, of course, occur in a much 
shorter time. The deadtime for an event 
lasting longer than the 0.3 s of memory 
is essentially equal to the 7.5-s write 
time. 

The 14th channel is used to record the 
time of day and for providing clock 
pulses for later use by the output 
digital-to-analog converter. The time of 
day is written on the tape in a pulse- 
coded magnetic (PCM) format. The accu- 
racy of this clock is ±1/2 s. The over- 
all format is custom-designed for the 
needs of this system. The general opera- 
tion of the recording side of the tape- 
triggering system is to digitize and 
record 13 channels of analog data. Fre- 
quency may be limited to 10,000 or 1,000 
Hz at the user's option. The worst case 
situation occurs with a 10,000-Hz signal. 
Even here eight samples are taken per 
cycle, insuring good analog resolution. 
The system is designed to record tran- 
sients, which appear on two to five chan- 
nels minumum, as selected by the user, 
during the memory time window so that 
spurious recording is eliminated. The 
system is designed to record only valid 
data and pack them on the tape as densely 
as possible by operating the tape 



recorder only when good data have been 
received. A pretrigger history is re- 
tained so that signals may be studied 
prior to the point that they exceed the 
threshold. The time of day is also 
recorded with each valid event so that 
the time history of the tape is pre- 
served. About 100 events can be stored on 
a 7,200-ft reel of tape. 

REPRODUCING SECTION 

Once the tape has been filled, or the 
desired period of monitoring is com- 
pleted, the output section of the tape 
triggering and recording system may be 
used to reproduce the digitally recorded 
signals in analog form. The basic func- 
tion of the output section is to read the 
digital signals from the tape and convert 
them to an analog output. The timing 
signals on channel 14 are used to control 
the digital-to-analog conversion rate so 
that the reproduced data look exactly 
like the recorded data. Fluctuations in 
tape speed or head skew will thus not 
degrade the data. The flexibility of the 
reproduction speeds on the tape recorder 
can be used to scale the analog output to 
match the speed of the recording device. 
One must, of course, keep track of these 
scaling factors so the proper time scale 
for the output records is known. 

The reproducing section uses compar- 
ators to read the serial bit stream for 
each channel and shift registers to 
assemble these bits into 12-bit words. 
As each 12-bit word is assembled, it is 



coupled to a digital-to-analog converter. 
After appropriate conditioning the analog 
signal for each channel is sent to its 
respective output connector, where it may 
be used to drive recording pens or 
oscillograph galvanometers. The output 
levels are ±10 V maximum. The present 
calibration factor is 1 V into the re- 
cording section equals 1.41 V out of the 
reproducing section. The pulse amplitude 
modulation type PCM format of the 14th 
channel contains a bit clock to operate 
the serial shift registers in the data 
channels, a word clock at one-twelfth the 
bit clock rate to operate the output 
latches, and the time of day information 
(fig. 3). The time of day is decoded and 
presented in a manner that is easy for 
the operator to read from graphic rec- 
ords. The output amplifier on the clock 
channel has only three possible states: 
-2 V, V, or +2 V. Positive excursions 
are read as binary coded data (BCD) ones, 
and negative excursions as BCD zeros. 
The overall time of day output as seen on 



a graphic recorder displays the hours, 
minutes, and seconds of the occurrence 
(fig. 4). 

The capacity of the tape system is 
being extended by implementing a biphase 
(Bi(j)L) encoding system as an option. 
This allows the use of direct recording, 
which means the tape can be run at much 
lower speeds for a given bandwidth. The 
NRZ data require the direct current (dc) 
levels provided by FM recording. By con- 
verting to biphase, the slower tape speed 
(15 ips) will at least quadruple the num- 
ber of events that can be recorded per 
tape. 

The specific parameters for the device 
discussed in this paper were chosen with 
a specific microseismic recording problem 
in mind (2^). Most of these parameters 
are readily variable to suit other appli- 
cations. The parameters were also re- 
stricted by the fact that the Bureau's 
recorders were already equipped with FM 



40-kHz 
reproduce 
clock 



Example clock_ 
digit = 9 I0 



PCM 
channel 



;o n p__m ra r 



V 



The PCM signal goes negative every 12 th clock pulse 
FIGURE 3. - Timing channel format. 



[o I i oj (o 




Seconds 



Minutes 



Hours 



Time = 17 h 34min 26s 
FIGURE 4. - Time of day information as presented on output record. 



electronics. There are obvious trade- 
offs between bandwidth and recording 
time. Total memory and prehistory memory 
length are obviously variable. By using 
direct-record electronics the record 
bandwidth could be greatly increased, 
though the coding would have to be 
changed to biphase since dc levels would 
be lost. Using direct record it is con- 
ceivable that a system with no dead time 



between events could be designed. The 
current system was designed to take ad- 
vantage of recorders the Bureau already 
possessed and to maintain compatibility 
with the normal analog use of these re- 
corders. The concept could be extended to 
use magnetic cartridge recorders or disc 
storage, though the system would have to 
be significantly modified. 



ENERGY ANALYZER 



The energy analyzer (fig. 5) was con- 
ceived and specified by the Denver Re- 
search Center of the Bureau of Mines. 
Design and construction were performed by 
the Rockwell International Energy Systems 
Group under a Bureau contract. Testing 
and final modification were performed by 
the Denver Research Center. The analyzer 
processes an externally supplied analog 
signal. On external command the unit 
measures the energy in a six-segment fre- 
quency distribution of the incoming sig- 
nal. The processed data are then made 
available to an external minicomputer. 

ANALOG-TO-DIGITAL CONVERSION 

The analyzer uses six passive low-pass 
analog filters to separate the signal 



into bands of frequency of interest and 
insures that no signal aliasing occurs. 
The analog signal is then converted to 
a digital signal, squared, scaled, 
and integrated for a selected time 
period. The external minicomputer-based 
microseismic system is then used to 
trigger, read status and data, and reset 
the analyzer. The unit has a built-in 
self test. Data are also readable from a 
front panel light-emitting diode (LED) 
display. 

The first stage of signal processing is 
to filter the analog signal. Filter 
bandwidths are each from near dc to 10 
kHz, 1.6 kHz, 0.8 kHz, 0.4 kHz, 0.2 kHz, 
and 0.1 kHz. The 10-kHz section is a 
full-bandwidth filter for the Bureau's 




FIGURE 5. - Energy analyzer system. 



microseismic application. The other sec- 
tions are designed to show possible 
energy distribution shifts as failure 
processes progress. The filters roll off 
at about 24 dB per octave with ultimate 
attenuation in excess of 36 dB. The 100- 
Hz filter section will thus attenuate any 
200-Hz components in the signal by 24 dB. 
These attenuations are obviously not 
infinite, but they are sufficient to sep- 
arate significant energy and for fre- 
quency shifts for the microseismic appli- 
cations for which they were built. 

The outputs of the analog filters are 
alternating-current-coupled to an analog 
multiplexer with sample and hold. Each 
of the six analog filter outputs is then 
converted to a digital signal using a 10- 
bit analog-to-digital converter. The 
sample rates of the highest to lowest 
frequency filter output are 2 13 , 2 12 , 
2 11 , 2 1 ", 2 9 , and 2 8 samples per 0.1 s. 
Considering the filter bandwidth, it can 
then be seen that the frequencies in each 
filter section are sampled at the rate of 
25.6 samples per cycle. The only excep- 
tion to this is the full bandwidth sec- 
tion, which is sampled at 8.2 samples per 
cycle at its upper end. This timing, 
which at first may seem awkward, has a 
very logical basis. The basic system 
clock is 163,840 Hz so each of the sample 
rates can be obtained by a simple divi- 
sion by 2. The highest frequency channel 
gets sampled every 2d clock pulse (81,920 
Hz), the next highest every 4th clock 
pulse, and this continues to form the 
previously mentioned sample rate. Simi- 
larly the integration segments can be 
normalized for direct comparison by sim- 
ply shifting left. The rates are chosen 
this high so that the sampling interval 
is very short, making integrals ob- 
tained by simple addition very good 
representations of exact integrals. The 
sample and hold can acquire a 10-V signal 
change in 350 ns to 0.01-pct accuracy. 

The analog-to-digital converter used in 
this system is a 12-bit unit, although 
only the 10 most significant bits are 
used in the calculation section. The 
signals from the filters are prescaled 
using voltage dividers so that the front 



panel input range is ±12 V. The maximum 
total throughput time of the analog- 
to-digital converter is 4 ys so it has no 
difficulty in maintaining the proper con- 
version rate for all six signal sections. 
The 10 bits that are used give a unipolar 
dynamic range of about 54 dB (9 data bits 
plus sign) . This dynamic range and the 
real-time conversion speed are maintained 
throughout the subsequent signal process- 
ing. If extending the dynamic range were 
desirable, an obvious way to accomplish 
this would be to precision-rectify the 
analog signal and use the converter in a 
unipolar fashion. The sampling scheme 
maintains a constant sample time to sig- 
nal period (AT/T) ratio (fig. 6) at the 
band limit. The highest frequency in 
each filter section is sampled at the 
same rate. For this system this sampling 
method offers no real advantage other 
than ensuring that the higher frequency 
sections get sampled adequately. A dis- 
advantage is that a signal whose fre- 
quency is low enough to allow it to pass 
through two or more filter sections will 
be sampled at diffferent rates in each 
section. This might yield slightly dif- 
ferent energy results for the same sig- 
nal. This problem is minimized because 
of the very high sample rates used in 
this system. If a set of bandpass fil- 
ters had been used to break the spectrum 
into nonoverlapping segments that were 




FIGURE 6. - Waveform digitization. 



not continuous, the constant AT/T sam- 
pling would be necessary to ensure equal 
accuracy for all segments. 

ARITHMETIC PROCESSING 

The next step in the energy processing 
sequence is to square each digitized val- 
ue. The original concept of the Bureau 
was to use one of the high-speed multi- 
plier integrated circuits that have 
recently become commercially available. 
Rockwell's approach was to use a program- 
able read only memory (PROM) look-up 
table. Both approaches are capable of 
the speed and dynamic range required. 
The Bureau's approach, however, offered 
much more flexibility since changes in 
word length and other system parameters 
could be easily accommodated. Using the 
Bureau's approach, the overall system 
operation would be simplified and accu- 
racy improved as well. The Rockwell 
scheme is complex and can perhaps best be 
understood by referring to table 1. 
Rockwell uses the 10-bit analog- 
to-digital output to form the address 
selector for the PROM's. The PROM's 
serve as a look-up table for the squared 
values. Each unique 10-bit analog-to- 
digital output has a corresponding 
squared value stored in PROM. The table 
uses minus voltages for illustration 
since the plus voltages are represented 
by their binary complement because of the 
sign bit. 

Rockwell chose to use the squared value 
rounded to the 16 highest bits in the 
PROM look-up table. The largest squared 
value that can be obtained with a 9-bit 
number is 18 bits, so some accuracy is 
sacrificed by rounding the 2 lowest bits. 
The Bureau's initial application calls 
for using only the 24 most significant 



bits of the 32-bit integrator output 
downstream at the squaring PROM's. The 2 
least significant bits of the square then 
have no contribution to these 24 highest 
bits. The actual error in accuracy as a 
percent of full scale for typical signal 
levels is also insignificant in terms of 
a percentage of true value. A 0.4-V root 
root mean square (RMS) signal would be 
in error about 2 pet over a typical inte- 
gration. It should, however, be clearly 
understood that this error is not inher- 
ent in this energy-measuring concept. It 
is an artifact of design decisions. If 
the Bureau's original concept of a 20 or 
more bit high-speed multiplier were used, 
or another 1,024.x 4-bit PROM were added, 
this error would be eliminated. 

After being processed by the squaring 
PROM's, the digital value is shifted left 
to normalize it to compensate for the 
different sample rates. Each sample 
interval-amplitude level pair defines a 
rectangular box, which approximates the 
signal increment for each AT (fig. 6). 
Shifting the signals makes the rectangu- 
lar box width equal for all filter sec- 
tions. Mathematically each shift is a 
division by 2. For example, the 1,600-Hz 
section (AT = 24.4 ys) values are shifted 
once to provide an effective AT of 
12.2 ys. Likewise, the 100-Hz section 
(AT = 390.6 ys) is shifted five times to 
provide an effective AT of 12.2 ys. 
Another way of looking at this is to 
realize that the 1,600-Hz section is 
sampled four times as often as the 100-Hz 
section so each of its samples is 
weighted at one-fourth the value of a 
100-Hz sample as it is added to form the 
integral approximation of its respective 
section. This makes the energy numbers 
from the various sections directly com- 
parable. For example, the energy numbers 



TABLE 1. - Internal energy processing J 



Input 
voltage 



Analog-to-digital 
(sign bit removed) 



Actual squared value 



PROM output 



12 

■8.49.. 
■2.34.. 
-.234.. 



11111111(511 10 ) 
101101001(361 10 ) 
001100100(100 10 ) 
000001010(10 10 ) 



11111111000000000(261,121 10 ) 
11111110100010001(130,321 1(J ) 
10011100010000(10,000) 
0001100100(100 10 ) 



1111111100000000(65,280) 
111111101000100(32,580) 
100111000100(2,500) 
00011001(25 10 ) 



Table is not exact because of rounding, 



from each section for a signal of low 
enough frequency to pass through all sec- 
tions will all be nearly equal, though 
the signal was actually sampled at dif- 
ferent rates in each section. 

The resultant 21-bit numbers from the 
shifter are then simply added for each 
respective filter section to provide the 
approximate integral of the squared sig- 
nal (fig. 7). The maximum integration 
period selectable is 1.5 s. The adder is 
32 bits wide to insure that no overflow 
will occur. Assume that the input was a 
sine wave at 6,000 Hz, 12 V peak-to-peak, 
and the integration period was 1.5 s. A 
sine wave has the property that the mean 
squared value is one half the peak value. 
The analog-to-digital converter would be 
full scale at the peaks. The average 
output of the squaring PROM would be 
216/2. The total number of samples of 
this average size would be 122,880. The 
output of the adder would then be 
4,026,531,840, which is less than the 
full-scale output of the adder (2 32 ). The 
integration period, which can range from 
0.1 to 1.5 s in 0.1-s increments, is 
selectable from front panel switches. 

The Bureau uses only the 24 most sig- 
nificant bits of the adder output in the 
present microseismic application, as was 
mentioned in the squaring PROM dis- 
cussion. Again it should be noted that 
this is not a limitation of the accuracy 
of the concept, only a design decision. 

APPLICATION 

The Bureau's Denver Research Center 
uses PDP 8/e data processors 3 in its rock 
burst-coal bump monitors. Since these 
processors have a 12-bit-wide bus, the 
upper 24 energy number bits are con- 
veniently read in 2 words. 

The rock burst monitor processes data 
from 24 channels, each of which is de- 
rived from a geophone at a unique loca- 
tion. One of these geophones is selected 

■^Reference to specific products does 
not imply endorsement by the Bureau of 
Mines. 



to be the energy channel. Its output is 
split between the rock burst monitor and 
the energy analyzer. Any active channel 
triggers the rock burst monitor. This 
trigger signal is then used to trigger 
the energy analyzer. A complication is 
introduced into this scheme since the 
rock burst monitor does not interrupt the 
computer unless certain valid event cri- 
teria are met within its logic. This by 
itself would leave the energy analyzer 
triggered on an invalid event and even- 
tually ready to output invalid data. The 
Bureau modified the energy analyzer's 
original trigger circuits so that when- 
ever a trigger occurs the energy analyzer 
is first reset and then triggered. This 
ensures that the energy analyzer and the 
rock burst monitor will both be process- 
ing the same event. Since the reset 
operation takes about 40 us, a delay 
circuit was introduced (fig. 8) so that 
the energy unit first has time to reset 
before it is triggered into a processing 
mode. The Bureau also found it necessary 
to introduce hysteresis into the energy 
analyzer trigger circuit to avoid spuri- 
ous noise triggers. 

Initial fieldwork using the energy 
analyzer has begun. The original filters 
were changed to 6.4, 3.2, 1.6, 0.8, 0.4, 
and 0.2 kHz for the rock burst monitoring 
system, since this provided more informa- 
tion on the spectral distribution of 
these particular signals. 

The motivation for this energy analysis 
stemmed from the promising application to 
ground failure prediction of the energy 
analysis done in the development of a 
roof fall warning system ( _4-_5 ) . Neither 
the energy analyzer nor the cruder 
techniques that preceded it have yet been 
successfully applied to rock burst pre- 
diction. The rock burst energy data are 
complicated by distance scaling effects, 
mine geometry effects, and rapid natural 
events mixed with manmade events during 
blasting. The scaling problem is being 
approached by taking ratios between the 
outputs of the various sections of the 
energy analyzer. The event-overlapping 
problem may be solvable by running the 
energy analyzer for fixed time intervals, 



10 



40-Hz 
clock 




40-Hz 
filter 



Amplifier 



6 V RMS 



Calibrate 



Channel Filter.Hz 
10,000 
I ,600 



800 
400 
200 
100 



Signal 
input 

± 12V peak 



Start pulse (§>_ 



Run 



6-channel 

low- pass 

filter 



10 kHz 



Sampling, 
rate, Hz 

81,920 
40,960 
20,480 
10,240 
5, I 20 
2,560 



1.6 kHz 



.8kHz 



■4kHz 



.2kHz 



.1 kHz 



Seguencer 
B 



Multiplexer 
address 



2 Data 

3 acguisition 

4 system 
5 



Sample strobe 



I63,840-Hz 
clock 



9 bits ■+ sigr 



Start switch 



Step switch — ' 



Reset switch 



Status out 
(reset) : 
from CPU 

Data out 
accepted : 
by CPU 



Timer 
selector 
switches 



Timer and 
control logic 



Sguaring 
PROMs 



Seguencer 
A 



Shift 
code 



LED panel 
ready 
display 



16 bits (rounded) 



0-to 5-bit 
parallel 
shifter 



\ Arithmetic 
section 



21 bits 



32-bit 
adder 



LED panel 
address 
display 



32 bits 
k- 



32 bits 



6-by-32-bit 

register 
TrJ state out 



Write 



[Bits 21 to 32 (most significant word) 



Bits 9 to 20 



Tristate 
drivers 



t->!2 bits data out 



Data status to CPU 



Word address code 4 



BCD time code 4 



-> Status in 



LED panel 
data display 



Status 

selector 

multiplexer 



Shift 

I 

2 
3 
4 
5 



Status 12-15 to CPU 
(central processor unit) 



FIGURE 7. - Energy analyzer arithmetic processing. 



11 




DATA (37)_ LT |5 
ACCEPTEDOUTL 



STATUS OUTL 



~i — rso^! 



FIGURE 8. - Energy system circuit modifications. 

similar to the approach used in the roof higher quality data than were previously 

warning system, instead of trying to available, but research must be done to 

establish a separate energy for each maximize its benefit, 
event. The energy analyzer provides 



CONCLUSIONS 



TAPE-TRIGGERING SYSTEM 



overall performance are exceeded by other 

transient recording devices, the device 

A tape triggering and recording device is a significant advance in the state of 

has been designed, constructed, and the art in terms of the number of data 

tested. While individual parts of the channels, the data bandwidth, memory 



12 



length, transient capture, and digital 
record-reproduce technology as grouped in 
one instrument (_b) . Indeed, before re- 
cent advances in performance and reduc- 
tions in cost of electronic circuits such 
as solid state memories, this device 
would have been impractical for indus- 
trial use (6). Since the trend in the 
electronics industry is toward a continu- 
ation of cost-performance improvement, 
the cost of similar units should drop 
while performance increases. Since the 
taped data are digitized, digital pro- 
cessing could be used as an alternate to, 
or in conjunction with, the present ana- 
log output. This device should have 
application in a wide variety of 
commercial recording problems. 

ENERGY ANALYZER 

The energy approximation produced by 
the energy analyzer is superior to those 
produced by any other devices the Bureau 
has used or examined. The hybrid digital 
technique of voltage-to-frequency conver- 
sion, for example, is limited because its 
frequency output varies with the ampli- 
tude of the voltage. Since the frequency 
output also indirectly sets an effective 
sample rate, each amplitude level is 
integrated with a differing degree of 
accuracy. At low amplitude, high signal 
frequency, the output can be a poor 
representation of the input integral. 



Analog techniques suffer from drift, lim- 
ited accuracy, and limited dynamic range. 
The all-digital approach used in the 
energy analyzer is a significant step 
forward in real-time energy processing. 
It is, however, recognized that at fre- 
quencies less than about 40,000 Hz, fast 
Fourier transform analyzers would provide 
a more detailed real-time energy analy- 
sis. The concept of the energy analyzer 
was to solve, in principle, the problem 
of real-time energy estimation throughout 
the range of frequencies used by acoustic 
emission researchers at a reasonable 
cost. With the present availability of 
video frequency analog-to-digital con- 
verters and very high speed digital mul- 
tipliers, it is felt that this has been 
accomplished. The motivation for a con- 
cise, few-number energy estimate stems 
from the Bureau's research in microseis- 
mic roof fall warning systems ^4-_5 ) • It 
has been found that having an approximate 
energy figure may be a key to being able 
to assess the stability of the rock mass. 
Often, for the Bureau's real-time stabil- 
ity assessments, a complete spectrum 
would be too much information. It is 
desirable to have a simple, compact means 
of presenting energy information to the 
on-site engineer. The energy analyzer 
and the principles it embodies provide a 
means for obtaining a good energy esti- 
mate that satisfies these criteria. 



REFERENCES 



1. Harris, D. D. , and R. L. Bell. The 
Measurement and Significance of Energy in 
Acoustic Emission Testing. Exp. Mech. , 
Sept. 1977, pp. 347-352. 

2. Sites, K. R. , and L. A. Millonzi. 
Tape Recording and Triggering Systems 
(contract HO282026, Science Applications, 
Inc.). BuMines OFR 116-81, 1980, 14 pp.; 
NTIS PB 81-243248. 

3. Blake, W. , F. Leighton, and W. I. 
Duvall. Microseismic Techniques for Mon- 
itoring the Behavior of Rock Structures. 
BuMines B 665, 1974, 65 pp. 

4. Fisher, C, Jr. Microseismic Roof 
Fall Warning System Development. Field 
-U.S. GOVERNMENT PRINTING OFFICE: 1983-705-020/96 



Trials and Commercial Prototype 
Fabrication. Volume II. Appendix C: 
Mine Data Collection Summary. Final 
Report (contract H0272029, Integrated 
Sciences Inc.). BuMines OFR 163(2)-81, 
1980, 228 pp.; NTIS PB 82-137845. 

5. Steblay, B. J. Progress in the 
Development of a Microseismic Roof Fall 
Warning System. Paper in Tenth Annual 
Institute on Coal Mining Health Safety 
and Research (Blacksburg, VA, Sept. 5-8, 
1979). Virginia Polytechnic Institute, 
1979, pp. 177-195. 

6. Nelson, R. Storage Oscilloscopes. 
Electronic Design News, June 10, 1981, 
pp. 76-88. 



INT.-BU.OF 



IES,PGH.,PA. 27222 













t.% y.^fe«x y.-afc^ yx&X. *^xy.^% *>• 



■>". .•->, 






^\ %/X»\ \/ •'£&'' \X.'«^ \„/ :]»•. V 








,/%. 



& V"X '. 



. X cF t o"'* "*b .A -*' 

^ k^°* ^IIP/ X°X, W$^i &*°* - 

v •:,*»- X .<y •»VL'4. *> V • »••» X a? 




>'..!•: 



.«!. *<< 



\ 






V<^ 



A <k *o.»» «G V V *'7Vs s A <> *o.* a «G V \9, *'.<,Y S A 

X&- % *•;&*.* X>^iX ^° -^<.°- ,A 







X*^>V %x„. . 



% <^. .A 



L> ^ 



^ 



/ A^X 



























o, ♦'TTi* A 



«^ <f>> 



%, *^f** A 




J -.y. v T4 *y 






^ * 






<> *••»• aG 




;\ 



^-. %># t« 



"oV 






* ^ 
» ^ 



-■!!?•./• %.*?z&\& v^^v./ "V'Js^v __ v< 



\y :jflK-. ^,# 






ft -^^ 



^ # -^Kx* + Wws J* \ 'SK* ^ ^ °w # ^ - -*8Kx** ^ °Ww ,^ 

V *^:t* a <^ *•.** <G^ % *7^t^* >v ^ ^t* «g* V *??7?« A ^ '*•* a° 



• ^ j 



^ * O H 



►*x 



•v 









» bJ^ * - 












: ; ^ ^ -if/ . ^ ^ . mm : y\ j -mes .^%n 






# ^ v 



X.^ 










* rt 



4y . 






* *> 







c°*..555S:^o >*"vJ 







•- X. ^ ;*. 



'bV 

3s? VV 










x^g^j^X 



- ""- 



• .^^ 




r X.^ •* 









dBlMiUtaialii 




■ 



■ I 




